A significant portion of the world""s methanol is produced by the catalytic reaction of synthesis gas obtained by the steam reforming of light hydrocarbons, particularly natural gas. Steam reforming of light hydrocarbons produces a synthesis gas containing hydrogen, carbon monoxide, and carbon dioxide wherein the synthesis gas composition may be characterized by a hydrogen-carbon oxide molar ratio defined as             [              H        2            ]        -          [              C        ⁢                  xe2x80x83                ⁢                  O          2                    ]                  [      CO      ]        +          [              CO        2            ]      
where [H2], [CO], and [CO2] are the mole fractions of the respective components in the synthesis gas. Methanol is formed from synthesis gas by the following reactions:
CO+2H2xe2x86x92CH3OH
CO2+3H2xe2x86x92CH3OH+H2O
In order to utilize the synthesis gas most efficiently in the above reactions, stoichiometric amounts of hydrogen and carbon oxides are preferred. Synthesis gas with a stoichiometric composition for methanol production has a value of the hydrogen-carbon oxide molar ratio of 2.0. Methanol is produced by reacting the synthesis gas catalytically in a pressurized reactor to yield methanol and unreacted synthesis gas, the methanol is condensed and separated from the unreacted synthesis gas, and a portion of the unreacted synthesis gas is recycled to the reactor feed to increase overall conversion. The remaining unreacted synthesis gas must be purged from the methanol reactor loop so that unreacted components do not build up in the reactor feed gas.
Synthesis gas produced by steam reforming of light hydrocarbons generally contains excess hydrogen when used for methanol production. This means that a significant amount of unreacted hydrogen must be withdrawn in the purge gas, which typically is used as fuel. This purge gas also contains valuable carbon oxides, which become unavailable for conversion to methanol, and this loss adversely affects methanol production economics.
Several approaches to this problem have been utilized in commercial methanol production. In one approach, imported carbon dioxide is mixed with either the synthesis gas feed to the methanol reactor or the feed hydrocarbon to the steam reforming step. This gives a methanol reactor feed gas that is closer to the preferred stoichiometric composition, but is possible only when a source of carbon dioxide is readily available. In another approach, unreacted synthesis gas is separated by various methods into a stream enriched in carbon oxides and a stream enriched in hydrogen, the carbon oxide-rich stream is recycled to the reformer or the methanol reactor, and the hydrogen-enriched stream is used for fuel. Membrane systems, absorption processes, and pressure swing adsorption have been used to effect separation of the unreacted synthesis gas.
An alternative approach is to generate the synthesis gas by methods other than steam reforming wherein these methods produce a synthesis gas closer to the preferred hydrogen-carbon oxide ratio for methanol production. Known methods to generate the preferred synthesis gas composition include the partial oxidation, autothermal reforming, and a two-stage process comprising steam reforming followed by oxygen secondary reforming. These methods all require a supply of oxygen, however, and the capital costs are higher than for simple steam reforming.
Steam reforming of light hydrocarbons continues to be a widely used process for generating synthesis gas for methanol production. It is desirable to develop methods for increasing the net conversion of the hydrocarbon feed to methanol product and for increasing the profitability of methanol production from this synthesis gas. In addition, it is desirable to reduce the cost of co-producing hydrogen and methanol from steam reformate to meet new or existing markets for these two products. The present invention, which is described below and defined by the claims which follow, offers an improved method for the co-production of hydrogen and methanol from synthesis gas generated by the steam reforming of light hydrocarbons.
The invention relates to a method for the production of methanol and hydrogen which comprises steam reforming a hydrocarbon-containing feed in a steam reforming zone to yield a synthesis gas comprising hydrogen, carbon monoxide, and carbon dioxide; introducing a first portion of the synthesis gas into a methanol synthesis zone to form methanol; reacting a second portion of the synthesis gas with steam to convert carbon monoxide to hydrogen and carbon dioxide to yield a shifted synthesis gas; cooling the shifted synthesis gas to yield a cooled shifted synthesis gas; separating the cooled shifted synthesis gas into a high-purity hydrogen product stream and a reject stream enriched in carbon dioxide; and introducing some or all of the reject stream into either or both of the steam reforming zone and the methanol synthesis zone. The hydrocarbon-containing feed may comprise one or more hydrocarbons containing from one to five carbon atoms. The hydrocarbon-containing feed may be natural gas.
The shifted synthesis gas may be separated by pressure swing adsorption. The reject stream from pressure swing adsorption may be introduced into the steam reforming zone or into the methanol synthesis zone.
The method may further comprise withdrawing from the methanol synthesis zone a crude methanol product and unreacted synthesis gas, withdrawing a first portion of the unreacted synthesis gas as purge, and recycling a second portion of the unreacted synthesis gas to the methanol synthesis zone.
The invention also relates to a method for the production of methanol and hydrogen which comprises steam reforming a hydrocarbon-containing feed in a steam reforming zone to form synthesis gas containing hydrogen, carbon monoxide, and carbon dioxide; converting a portion of the synthesis gas to methanol in a methanol synthesis zone; withdrawing from the methanol synthesis zone a crude methanol product and unreacted synthesis gas; recycling a first portion of the unreacted synthesis gas to the methanol synthesis zone; separating a second portion of the unreacted synthesis gas to yield a high-purity hydrogen product stream and a reject stream; recycling a first portion of the reject stream to either or both of the steam reforming zone and the methanol synthesis zone; and utilizing the second portion of the reject stream as fuel to provide heat to the steam reforming zone. The hydrocarbon-containing feed may comprise one or more hydrocarbons containing from one to five carbon atoms. The hydrocarbon-containing feed may be natural gas.
The second portion of the unreacted synthesis gas may be separated by pressure swing adsorption. The first portion of the reject stream may be introduced into the steam reforming zone or the methanol synthesis zone.